1. Field of the Invention
The present invention relates to a hydrostatic gas bearing, a rotator and a CT scanner, and more particularly, it relates to a hydrostatic gas bearing supporting a rotor, a rotator supported by this hydrostatic gas bearing and a CT scanner employing this rotator.
2. Description of the Background Art
In the field of roller bearings generally used for supporting rotors, hydrostatic gas bearings are increasingly employed in order to implement high-speed/high-precision rotation, low noise, low vibration, long life etc.
For example, a CT scanner in the field of medical diagnostic image formation has a rotating shaft loaded with a radiation source or the like. National Patent Publication Gazette No. 2004-528113 discloses an example of supporting a rotating shaft of a CT scanner with a hydrostatic gas bearing in order to 1) increase the speed of the CT scanner for reducing the scanning time, 2) obtain a silent operation of the scanner unit, and 3) reduce the maintenance frequency by preventing a contact portion from wearing.
National Patent Publication Gazette No. 2004-528113 describes such a structure that a rotating gantry has an annular bearing race on the outer periphery thereof so that the rotating gantry is supported on the bearing race in a noncontact manner by a plurality of hydrostatic bearing pads held on a fixed gantry. According to this structure, a radial bearing pad supports a polished outer peripheral surface on the outer side of the bearing race, and an axial bearing pad supports flat axial surfaces from the both sides of the bearing race.
A radial ball stud is set at the back (opposite to the side facing the rotating shaft) of the radial bearing pad, and a spring element is provided between the radial bearing pad and the ball stud. The radial bearing pad is arranged in the vicinity of the outer periphery of the bearing race, and held on a prescribed position by the radial ball stud. In other words, the radial ball stud is tightened from behind up to desired tension in the radial direction of the rotating shaft through the spring element, and the radial position of the radial bearing pad is controlled by adjusting the radial ball stud.
According to National Patent Publication Gazette No. 2004-528113, the ball stud is adjusted in a direction of a straight line connecting the center of rotation of the rotating shaft and the forward end of the ball stud with each other. In other words, the ball stud moves the radial bearing pad in a direction parallel to the direction connecting the center of rotation of the rotating shaft and the forward end of the ball stud with each other. Therefore, the quantity of position control of the ball stud directly decides the position of the radial bearing pad arranged in front of the ball stud with respect to the rotating shaft, i.e., the quantity of a bearing clearance (clearance between the radial bearing pad and the rotating shaft).
However, this bearing clearance is so small (generally not more than 1 mm) that the ball stud must be finely adjusted in order to adjust the quantity of the bearing clearance. Particularly in a used state of a rotator disclosed in National Patent Publication Gazette No. 2004-528113, i.e., in such a state that the axial direction of the rotor is arranged substantially horizontal, the weight of the rotating shaft is directly applied to the radial bearing pad arranged under the rotating shaft. In this state, it is extremely difficult to adjust the bearing clearance to an ideal quantity with a screw or the like mounted on the outer periphery of the ball stud at the back of the radial bearing pad.
In the rotator, the rotor is desirably as compact and lightweight as possible, in consideration of the load capacity of the hydrostatic gas bearing, the driving force of a motor rotating the rotor and the cost for the rotator.
In the rotator described in National Patent Publication Gazette No. 2004-528113, however, the radial bearing pad supports the outer peripheral surface of the bearing race, and hence the bearing race must have a sufficient width (thickness) to be supported by the radial bearing pad. Therefore, the thickness of the outer diametral portion of the rotor is increased, and the weight of the rotor, the moment of inertia around the rotating shaft in rotation and the driving force necessary for the rotation are also increased. Further, the radial bearing pad supports the rotor on the outer diametral portion of the bearing race, and hence the size of the overall rotator is also increased.